Transceivers based on optical fibers allow the transmission of signals over long distances. They conventionally use frequency multiplexing so as to be able to transmit/receive several signals with a single optical fiber. They therefore transmit signals in a wide band of frequencies.
However, today's coupling devices are in general tailored to suit a restricted range of frequencies.
Moreover, an optical signal travelling in a conventional optical fiber is polarized in a random manner, that is to say the orientation of its electric field is random. Also, an optical signal travelling in a conventional waveguide of an integrated circuit, that is to say a waveguide of rectangular cross section, allows polarization of the light signal in two directions only. The first direction, called transverse electric (TE polarization), is defined as parallel to the layers of the integrated circuit, for example parallel to the buried insulating layer in technologies of silicon on insulator type. The second direction, called transverse magnetic (TM polarization), is defined as being orthogonal to the first direction. Certain photonic hardware components are particularly suitable for signals polarized in a transverse electric manner, and other photonic hardware components are particularly suitable for signals polarized in a transverse magnetic manner. Other hardware components can receive signals polarized in either way.
Various means exist for coupling an input/output device to an integrated circuit.
A first solution consists in coupling the input/output device on the upper face of the integrated circuit, and of transmitting the signal to the waveguide by way of a grating-type coupler.
This solution makes it possible to transmit the transverse electric component of the optical signal as well as the transverse magnetic component via its conversion into a transverse electric component. However, this transmission is done only over a very small optical bandwidth. Consequently a non-negligible part of the optical power, sometimes greater than 50%, may be lost.
A second solution consists in coupling the input/output device on a lateral face of the integrated circuit and makes it possible to transmit a signal polarized in the transverse electric direction and in the transverse magnetic direction.
However, the existing solutions are expensive since they require the implementation of specific methods of fabrication, and they do not allow sufficient confinement of the transverse magnetic component, thus giving rise to diffusion of the signal into the carrier substrate of the integrated circuit and therefore optical power losses.
It is therefore desirable to limit the diffusion of the transverse magnetic component into the carrier substrate.